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Δίκτυα Υπολογιστών II Channel separation in 802.11b networks Power Frequency Channel 1Channel 6Channel 11 25 MHz More channels at the same time => severe spectral overlapping 3 channels can be used at the same time in the same area Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II IEEE 802.11a/g This physical layer implementation is based on OFDM (Orthogonal Frequency Division Multiplexing). The information is carried over the radio medium using orthogonal subcarriers. A channel (16.25 MHz wide) is divided into 52 subcarriers (48 subcarriers for data and 4 subcarriers serving as pilot signals). Subcarriers are modulated using BPSK, QPSK, 16-QAM, or 64-QAM, and coded using convolutional codes (R = 1/2, 2/3, and 3/4), depending on the data rate. Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II Frequency domain Presentation of subcarriers in frequency domain: 52 subcarriers Frequency 16.25 MHz By using pilot subcarriers (-21, -7, 7 and 21) as a reference for phase and amplitude, the 802.11a/g receiver can demodulate the data in the other subcarriers. Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II Time domain Presentation of OFDM signal in time domain: Time Guard time for preventing intersymbol interference In the receiver, FFT is calculated only during this time Symbol duration Next symbol 4.0  s 3.2  s0.8  s Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II Bit-to-symbol mapping in 16-QAM Gray bit-to-symbol mapping is usually used in QAM systems. The reason: it is optimal in the sense that a symbol error (involving adjacent points in the QAM signal constellation) results in a single bit error. 0000010011001000 0001010111011001 0011011111111011 0010011011101010 Example for 16-QAM Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II Orthogonality between subcarriers (1) Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Subcarrier n+1 Previous symbol Next symbol Orthogonality over this interval Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II Orthogonality between subcarriers (2) Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Subcarrier n+1 Previous symbol Next symbol Orthogonality over this interval Each subcarrier has an integer number of cycles in the FFT calculation interval (in our case 3 and 4 cycles). If this condition is valid, the spectrum of a subchannel contains spectral nulls at all other subcarrier frequencies. Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II Multipath effect on subcarrier n (2) Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Delayed replicas of subcarrier n Guard time not exceeded: Delayed multipath replicas do not affect the orthogonality behavior of the subcarrier in frequency domain. There are still spectral nulls at other subcarrier frequencies. Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II Multipath effect on subcarrier n (3) Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Delayed replicas of subcarrier n Mathematical explanation: Sum of sinusoids (with the same frequency but with different magnitudes and phases) = still a pure sinusoid with the same frequency (and with resultant magnitude and phase). Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II Multipath effect on subcarrier n (4) Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Replicas with large delay Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II Multipath effect on subcarrier n (5) Guard time Symbol part that is used for FFT calculation at receiver Subcarrier n Previous symbol Next symbol Replicas with large delay Guard time exceeded: Delayed multipath replicas affect the orthogonality behavior of the subchannels in frequency domain. There are no more spectral nulls at other subcarrier frequencies => this causes inter-carrier interference. Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II IEEE 802.11a in Europe 802.11a was designed in the USA. In Europe, a similar WLAN system – HiperLAN2 – was designed by ETSI (European Telecommunications Standards Institute), intended to be used in the same frequency band (5 GHz). Although HiperLAN2 has not (yet) took off, 802.11a devices, when being used in Europe, must include two HiperLAN2 features not required in the USA: DFS (Dynamic Frequency Selection) TPC (Transmit Power Control) Δρ. Γεώργιος Δημητρακόπουλος

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Δίκτυα Υπολογιστών II IEEE 802.11g PHY 802.11g is also based on OFDM (and same parameters as 802.11a). However, 802.11g uses the 2.4 GHz frequency band, like 802.11b (usually: dual mode devices). Since the bandwidth of a 802.11b signal is 22 MHz and that of a 802.11g signal is 16.25 MHz, 802.11g can easily use the same channel structure as 802.11b (i.e. at most three channels at the same time in the same area). 802.11g and 802.11b stations must be able to share the same channels in the 2.4 GHz frequency band => interworking required. Δρ. Γεώργιος Δημητρακόπουλος